Re: [RFC PATCH v15 6/7] sched: Fix proxy/current (push,pull)ability

Peter Zijlstra
2025-03-17T14:08:05Z

On Wed, Mar 12, 2025 at 03:11:36PM -0700, John Stultz wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b4f7b14f62a24..3596244f613f8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6722,6 +6722,23 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
> }
> #endif /* SCHED_PROXY_EXEC */
>
> +static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
> +{
> + if (!sched_proxy_exec())
> + return;
> + /*
> + * pick_next_task() calls set_next_task() on the chosen task
> + * at some point, which ensures it is not push/pullable.
> + * However, the chosen/donor task *and* the mutex owner form an
> + * atomic pair wrt push/pull.
> + *
> + * Make sure owner we run is not pushable. Unfortunately we can
> + * only deal with that by means of a dequeue/enqueue cycle. :-/
> + */
> + dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
> + enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
> +}

So this is probably fine at this point; but we should eventually look at
fixing this.

We can probably look at (ab)using sched_class::set_cpus_allowed() for
this.